On the Tip of Your Tongue. (Idiom) If something is on the tip of your tongue, you know you know what it is, but you just can't quite remember it at the moment.

Where have I seen Rachel McAdams before? I just can't quite place my finger on it. She looks exceedingly familiar; it's right on the tip of my tongue. Hmmmmm. It's killing me. Damn! Where have I seen that face before!?

Oh oh oh oh oh oh! I remember. I remember: